1. Field of the Invention
The present invention relates to a temperature monitor of a wire electric discharge machine. More specifically, the present invention relates to a temperature monitor of a wire electric discharge machine that monitors the temperature of a working fluid or the temperature around an electric discharge machine (outside air temperature), displays an alarm when the monitored working fluid temperature or outside air temperature is outside a predetermined range, and calls off the alarm after the monitored temperature returns to within the predetermined range and a temperature difference between the working fluid and a workpiece or a temperature difference between the outside air temperature and the electric discharge machine body disappears.
2. Description of the Related Art
A wire electric discharge machine machines a workpiece into a desired shape by causing a discharge by applying a voltage to a machining gap between a wire electrode and a workpiece and at the same time, changing a relative position of the wire electrode with respect to the workpiece. In general, the dimensional precision, perpendicularity, or precision of angle is required from machining results of a workpiece.
In wire electric discharge machining, the required machining precision changes depending on conditions such as the material, thickness and the like of a workpiece and in some cases, high precision of a few micrometers is required. Thus, if the temperature of the working fluid or around the wire electric discharge machine changes and the workpiece or the electric discharge machine body is deformed during machining, the machining precision deteriorates and the dimension error may become larger than an allowable value. Therefore, it is important to control the working fluid temperature or the temperature around the electric discharge machine to obtain a desired machining result from the electric discharge machine.
Regarding the temperature control of electric discharge machining, Japanese Patent Application Laid-Open No. 4-82624 discloses a technology of machining while controlling the working fluid temperature and the outside air temperature to within a predetermined temperature range by managing the working fluid temperature and the temperature around an electric discharge machine (outside air temperature). The technology relate to controlling of working fluid temperature and temperature around an electric discharge machine while the electric discharge machine is in operation.
A case where a wire electric discharge machine restarts to work after pausing for a predetermined time will be considered. It is assumed that the wire electric discharge machine is without human intervention during pausing and no machine operation is performed by humans. It is also assumed that the temperature control unit of the working fluid and air conditioner stop while the wire electric discharge machine pauses and the wire electric discharge machine operates with the minimum power to maintain the machine.
First, the temperature change of the working fluid will be considered.
If the temperature around the electric discharge machine (outside air temperature) is lower than a control target temperature of the working fluid in a normal operation state (such control target temperature is hereinafter referred to as control temperature), the temperature of the working fluid gets closer to the outside air temperature when the electric discharge machine pauses and after the pause state ends and the electric discharge machine restarts to work, the temperature of the working fluid is controlled to a control temperature. In the meantime, the state of the electric discharge machine is classified into the following four states (See FIG. 9). “Control temperature” below means the optimum control target temperature to machine a workpiece by a wire electric discharge machine.
A: A normal operation state in which the working fluid temperature is maintained at control temperature
B: A pause state in which the temperature control unit of the working fluid stops
C: A preparation state to restart an operation from the pause state in which the temperature control of the working fluid is restarted by the temperature control unit and a working fluid circulating pump
D: A state immediately after the working fluid temperature becomes equal to the control temperature in which there is a temperature difference between the workpiece and working fluid
E: A state returning to the normal operation state after the temperature difference between the workpiece and working fluid disappears (this state E is the same as the above state A).
If, in the operation restart preparation state C described above, machining is started before the temperature of the working fluid reaches the control temperature, the working fluid temperature will be different before machining and after machining. The workpiece is deformed as the temperature of the working fluid changes, causing defective machining.
Here, application of the technology described in Japanese Patent Application Laid-Open No. 4-82624 described above to the above restart preparation state C is considered.
According to the air-conditioning management method of a wire electric discharge machine described in the above Patent Document, an alarm signal is issued if the working fluid temperature or the room temperature is not within a predetermined temperature range when a predetermined time passes after power-on and the electric discharge machine is activated to start an operation if the working fluid temperature and the room temperature are within the predetermined temperature range. However, this air-conditioning management method issues an alarm warning to determine whether a working fluid cooling apparatus or an air conditioner is defective or not. Moreover, according to the air-conditioning management method, machining is immediately started when the working fluid temperature or the room temperature falls within the predetermined temperature range. That is, the technology is not intended to start an operation after waiting until a temperature difference between the electric discharge machine or workpiece and the working fluid or outside air disappears.
In addition, an abnormal condition display system of the working fluid temperature of a wire electric discharge machine that uses two temperature sensors and displays an alarm or stops machining when the working fluid temperature falls outside the predetermined temperature range due to a failure of cooler or an operator's failure to press a switch is disclosed in Japanese Patent Application Laid-Open No. 2-116418. If the technology of the abnormal condition display system of the working fluid temperature is used, an alarm is raised if an attempt is made to start machining when the working fluid temperature is outside the predetermined temperature range like the above operation restart preparation state C and therefore, machining can be performed only when the working fluid temperature is within the predetermined temperature range and the working fluid temperature does not change during machining.
Even if the technology disclosed in Japanese Patent Application Laid-Open No. 2-116418 described above is used, the workpiece or electric discharge machine body may thermally be deformed during machining. For example, immediately after the working fluid temperature or the ambient temperature of the wire electric discharge machine (outside air temperature) returns from outside the predetermined temperature range to inside the predetermined temperature range, there may be a temperature difference between the working fluid temperature or outside air temperature and the workpiece or electric discharge machine body. Though the temperature difference decreases and disappears over time, but if electric discharge machining is started before the temperature difference disappears, the temperature of the workpiece or electric discharge machine body will change during machining so that thermal deformation may arise due to the change of temperature of the workpiece during machining.
FIG. 10 is a diagram schematically showing temperature changes of the working fluid and workpiece when a wire electric discharge machine restarts to operate from a state of pause.
In FIG. 10, a solid line represents the work fluid temperature and a broken line represents the temperature of the workpiece. Characteristics such as thermal conductivity are different in the working fluid and the workpiece and thus, when the wire electric discharge machine restarts to operate, times necessary for respective temperatures to reach a set temperature are different. That is, in FIG. 10, while the working fluid temperature reaches the set temperature at tc, the temperature of the workpiece reaches the set temperature at tE. If, for example, electric discharge machining is started at time tE(tc<tD<tE), the temperature of the workpiece changes during machining and thermal deformation is caused, leading to defective machining.
Similar consideration also applies to the change of outside air temperature around the electric discharge machine. If machining of the workpiece is started after an air conditioner is restarted to operate from the state of pause and before the ambient temperature of the wire electric discharge (outside air temperature) reaches the set temperature, the temperature of the machine body of the wire electric discharge machine is different between when machining is started and when machining is finished. This means that the machine itself is thermally deformed with temperature changes of the outside air temperature, causing defective machining.
Thus, a conventional wire electric discharge machine is configured to start machining even in a state in which the temperature of a workpiece or electric discharge machine body can change. This can cause defective machining, posing a problem of declining efficiency with an increasing number of man-hours due to repeated machining.